It is a usual property of cardiac pacemakers that they can stimulate a heart to perform a contraction by means of electrical stimulation pulses. Depending on the respective structure, setting or programming of the cardiac pacemaker, stimulation pulses are delivered unconditionally at a time which is predetermined by means of a timer—this corresponds to a triggered mode of the pacemaker—or conditionally, that is to say only when no natural contraction of the heart occurs within a predetermined time window (which is usually referred to as the escape interval). The last-mentioned operating mode of the cardiac pacemaker is referred to as the inhibited mode because the detection of a natural contraction of the heart (which can be recognized as a so-called intrinsic event in an electrocardiogram (ECG) within the escape interval leads to inhibition of the delivery of a stimulation pulse.
In regard to the escape intervals, a distinction is drawn for example between atrial and ventricular escape intervals, more specifically depending on whether this involves triggering atrial stimulation pulses for delivery to the atrium of a heart or triggering of ventricular stimulation pulses which are to be delivered to the ventricle. In particular, the delivery of ventricular stimulation pulses, in so-called AV sequential cardiac pacemakers, is controlled in such a way that a ventricular escape interval (also referred to as the AV interval) is started by an atrial event. The ventricular escape or AV interval is of a duration which approximately corresponds to the duration of a natural stimulation conduction from the atrium to the ventricle. Frequently the AV interval is set somewhat longer in order to prevent a stimulation pulse preceding a natural contraction or indeed coinciding with a simultaneous natural contraction. In the last-mentioned case (coincidence of natural and stimulated contraction), that is referred to as fusion events (referred to in English as fusion beats). Such a prolongation of the AV interval is also referred to as hysteresis.
Besides natural transconduction of stimuli from the atrium to the ventricle, there may also be a stimulus conduction from the ventricle to the atrium, which is referred to as retrograde, with the consequence that a ventricular contraction triggers an atrial contraction by retrograde stimulus conduction with a corresponding delay. In general such an atrial event which is based on retrograde conduction is premature from physiological points of view and, in connection with cardiac pacemakers, can result in a pacemaker-induced tachycardia, also referred to as pacemaker-mediated tachycardia or PMT.
A further problem is that the far-field of a ventricular stimulus is also to be perceived (detected) in the atrium of a heart. The result of this can be that far-field detection of a ventricular stimulation pulse in the atrium is falsely evaluated as an intrinsic atrial event, that is to say which is involved with a natural contraction. That problem is usually combated by a post-ventricular, atrial refractory time which begins with the triggering of a ventricular stimulus and within which no atrial events are detected or, if admittedly detected, are then at least not evaluated.
Electrostimulators such as, in particular, cardiac pacemakers produce a muscle contraction by the delivery of a sufficiently strong stimulation pulse. In the case of implantable cardiac pacemakers, it is basically desirable if the respective stimulation pulse is just sufficiently strong to sufficiently stimulate the myocardium in the area around the stimulation electrode. That stimulus which results in contraction of the myocardium is then propagated in a natural fashion starting from the stimulation point and accordingly results in complete contraction of the respective heart chamber, that is to say atrium or ventricle. That stimulation pulse amplitude which is just sufficient to cause response (capture) of the myocardium to a stimulation pulse is referred to as the stimulation threshold. A stimulation pulse above the stimulation threshold, which causes a contraction of the myocardium, is referred to as superthreshold while a stimulation pulse which does not produce a contraction of the myocardium, below the stimulation threshold, is referred to as subthreshold.
Stimulation pulses which are just of a pulse strength which for safety reasons is above the stimulus threshold require less energy than even stronger stimulation pulses. In consideration of the energy capacity of implanted cardiac pacemakers, which is limited from the outset, the aim is always to detect the respective stimulus threshold at the stimulation location as precisely as possible in order to be able to adapt the pulse strength or amplitude of a stimulation pulse to the respectively current stimulus threshold as precisely as possible. The stimulus threshold can change in the course of time and therefore a one-off stimulus threshold test upon implantation is generally not sufficient.
It is therefore known for cardiac pacemakers to be provided with means for stimulation success checking, with which it is possible to determine whether a delivered stimulation pulse has resulted in a response on the part of the myocardium and has produced a corresponding stimulus response.
(Direct) automatic determination of the presence (absence) of a stimulus response after a stimulation pulse by virtue of measurement of the evoked potential with the same electrode as that with which the stimulation pulse whose effectiveness is to be checked was delivered has already been implemented for ventricular stimulation pulses. The corresponding principles of such a solution can in principle also apply to the atrium level.
In actual fact however at the present time, for the atrium region, there is no implantable or external system with which fully automatic atrial threshold tracking or fully automatic atrial stimulus threshold determination can be reliably effected.
The lack of availability of a corresponding system for atrial application is no surprise because the evoked potentials in the atrium, by virtue of its muscle mass which is substantially less (in comparison with the ventricle) and because of the differences in the specific stimulus conduction system, are of a signal quality which is substantially worse for detection, than the evoked ventricle potentials.
Approaches for providing an atrial stimulus response or capture detector are known from U.S. Pat. Nos. 5,601,615, 5,861,012 and 6,584,354. Those known approaches are unsatisfactory for various reasons.
Therefore, it is desirable to provide a cardiac pacemaker affording stimulation success checking which is also suitable for the atrium of a heart and which, as far as possible, avoids disadvantages of the state of the art.